The invention relates to a method and an apparatus for detecting collisions in printing machines.
Printing machines typically have a number of rotationally driveable rotary bodies, each of which can be adjusted in a direction normal to the axis of rotation by means of an associated actuator. For example, in a typical flexographic printing machine, a plurality of inking units are arranged at a common reaction cylinder, and each inking unit has two such rotary bodies, i.e. a printing cylinder and an inking roller. During the printing operation the inking roller is in rolling engagement with the printing cylinder, and the printing cylinder itself is in rolling engagement with the printing medium which is guided around the reaction cylinder, so that the ink is transferred from the inking roller to the printing parts of the printing blocks of the printing cylinder and, then, a corresponding printed image is formed. In case of maintenance and retooling, for example, when the cylinders are exchanged, the inking roller is separated from the printing cylinder, and the printing cylinder is separated from the reaction cylinder. To this end, the inking roller and the printing cylinder are moved (adjusted) in a substantially radial direction relative to the reaction cylinder by means of their respective actuators. In this case there is a risk that the inking roller and the printing cylinder collide with one another or with other machine components, so that damages are caused.
It is therefore common practice to provide a monitoring system for detecting such collisions and for stopping the corresponding actuators immediately, in order to avoid damages or injuries of the operating personnel.
In conventional printing machines the detection of collisions is achieved by monitoring the driving torques of the actuator motors. When the rotary body hits an obstacle during the adjustment process, the driving torque transmitted from the actuator motor is increased, and when this driving torque exceeds a certain threshold value, this indicates that a collision has occurred, and the actuator motor is stopped.
The actuator, e.g. a spindle drive, generally has a large transmission ratio, so that even a comparatively small torque of the drive motor generates a high actuating force. Conversely, this means that the increase of the resistance opposing the adjusting movement in case of a collision leads to only a comparatively small increase in the transmitted torque. The collision detection system is therefore relatively slow and inaccurate. Although, in principle, the sensitivity can be increased by lowering the threshold value at which the actuator motor is stopped, this threshold value must always be selected so high that the sometimes considerably high frictional forces which occur during the adjusting movement can be overcome.